1. Field of the Invention
The present invention relates to a pitch modification method by glottal closure interval extrapolation, and particularly when concatenating original speech segments to synthesize speech, a pitch modification method which is capable of modifying pitches of the speech signals by the glottal closure interval extrapolation, while maintaining a very good quality in the modified speech.
2. Description of Related Art
Generally, speech synthesis method is classified into limited vocabulary synthesis method and non-limited vocabulary synthesis method. Formant, linear prediction coefficient (LPC), line spectrum pair (LSP) etc. of a parameter type in the non-limited vocabulary synthesis method, have been studied, these methods have a little poor quality, but have the advantage of making a variety of synthetic sounds by modifying sound source and vocal tract parameter etc. To obtain synthetic sounds of the very good quality, a pitch synchronous overlap and add (PSOLA) method has been studied as a typical scheme which varys pitches in time domain to concatenate original speech segments.
FIGS. 1A to 1F are waveforms showing steps of pitch modification by the prior art PSOLA method.
FIG. 1A is a waveform of a speech signal X(t), FIGS. 1B and 1C are waveforms of weight functions W.sub.1 (t) and W.sub.2 (t), and FIG. 1D is a waveform of a speech signal X.sub.1 (t) obtained by multiplication of the speech signal X(t) and the weight function W.sub.1 (t). FIG. 1E is a waveform of a speech signal X.sub.2 (t) obtained by multiplication of the speech signal X(t) and the weight function W.sub.2 (t), and FIG. 1F is a waveform of a speech signal Y(t) varying a pitch by overlapping of the speech signal X.sub.1 (t) and the speech signal X.sub.2 (t) as shown in FIGS. 1D and 1E.
The prior art PSOLA method includes first step of generating a first speech signal by multiplying the original speech signal by a first weight signal, second step of generating a second speech signal by multiplying the original speech signal by a second weight signal, and third step of overlapping and adding the first speech signal and the second speech signal in a desired pitch length to generate a pitch-changed speech signal.
The prior art PSOLA method is explained with reference to FIGS. 1A to 1F.
First, the original speech signal X(t) shown in FIG. 1A is multiplied by the first weight signal W.sub.1 (t) shown in FIG. 1B to generate the first speech signal X.sub.1 (t) shown in FIG. 1D, and the original speech signal X(t) shown in FIG. 1A is multiplied by the second weight signal W.sub.2 (t) shown in FIG. 1C to generate the second speech signal X.sub.2 (t) shown in FIG. 1E.
Then, the first speech signal X.sub.1 (t) and the second speech signal X.sub.2 (t) are overlapped and added in the desired pitch length to generate the pitch-changed speech signal Y(t).
Since the prior art PSOLA method has large effect of window which is applied by pitch unit according to increase of pitch modification rate and large spectrum distortion generated by overlap and add of two weighted speech signals, articulation of the synthetic speech is deteriorated.